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Description/Abstract

The influence of non-linearities on wave-induced motions and loads has been thefocus of many investigations in the past few years and continues to be an importantissue. A number of two- and three-dimensional methodologies have been developed,by and large, partly accounting for various non-linearities. Non-linear radiation, andto an extent diffraction, is the main problem and its solution via a three-dimensionalmethod using Eulerian-Lagrangian schemes is likely to be complex and timeconsuming for practical applications. On the other hand two-dimensional methods, inspite of issues associated with accounting for forward speed, offer more possibilitiesof making practical advances in dealing with non-linearities.

A two-dimensional hydroelasticity analysis for symmetric (i.e. vertical motions,distortions and loads) dynamic behaviour in waves, including the influence of nonlinearities,is presented in this thesis using two methods. In the first method the totalresponse is decomposed into linear and non-linear parts. The linear part is evaluatedusing the conventional two-dimensional linear hydroelasticity analysis. The nonlinearhydrodynamic forces are due to changes in added mass and dampingcoefficients, as well as restoring and incident wave forces, all evaluated over theinstantaneous wetted surface. Non-linear forces due to slamming (bottom impact andflare) and green water (treated in a quasi-static manner) are also added. One aim ofthe thesis is to investigate the influence/importance of each of the non-linearhydrodynamic forces. Furthermore, the effects of assumptions made when usingthese hydrodynamic forces, e.g. frequency dependence of added mass, neglecting thedamping coefficients in some components and evaluation of derivatives, areinvestigated. The solution in the time domain is obtained using direct integration andconvolution integration, the latter based on the impulse response functions of the hullin its mean wetted surface. In the second method the response, including nonlinearities,is obtained from the solution of one system of equations of motion, wherethe added mass and damping coefficients and the restoring, incident wave anddiffraction forces are evaluated at the instantaneous draft. Non-linear forces due toslamming (bottom impact and flare) and green water (treated in a quasi-staticmanner) are also added.

Both methods are applied to the S-175 containership, for which experimentalmeasurements of motions and loads in large amplitude regular head waves areavailable. Comparisons made between predictions and measurements (heave andpitch motions, vertical acceleration and vertical bending moment) indicate goodoverall agreement. The comparisons also show that the influence of flare slamming isimportant for the range of speeds and wave amplitudes investigated.